Through the evolution of storage type hydroelectric generating stations which operate at the above pressure ratios it was discovered that at deep submersion, i.e. location of the pump-turbine beneath the lowest water level, which is technically and economically mostly unacceptable, but is necessary for the elimination of cavitation, no reversible single-stage pump-turbine can generally by applied as an independent unit. The reason for this lies in the fact that such a unit cannot be constructed in a manner such that on the one hand the submersion will be acceptable and on the other hand the plant will not be endangered by cavitation, particularly during operation as a storing machine at the given pressure ranges.
In order to overcome these disadvantages, the known embodiments use two-stage pump-turbines with equally divided pressure ratios to each stage. These plants operate satisfactorily with regard to cavitation however, if the first stage operates at a lower pressure, cavitation will occur because the first stage will be driven faster than normal for its pressure by the higher pressure second stage. However, there still exists the disadvantage that no known two-stage plant operating at a pressure usually divided into equal parts also enables obtaining optimum efficiency in both operation as a turbine for driving a generator and as a pump for filling a reservoir. In addition to known reversible two-stage plants there have been proposed multi-stage irreversible plants. The disadvantage of these plants lies in the fact that in order to assure moderate submersion there has to be added a supplying pump to the storing pump. The supplying pump is mounted on the axle of the turbine or storing pump, or it is provided with its own electric drive. A disadvantage of the first embodiment is a lower efficiency of the plant because that together with the main hydraulic pump-turbine, there also rotates the rotor of the supplying pump, the latter rotation actually being useless. A disadvantage of the second embodiment is the arrangement of a separate pump with own drive in circumstances in which the construction of the plant should be compact, and not require more extensive construction entailing higher cost.